The invention relates to a balance shaft unit for reciprocating machines, consisting of a balance shaft supported in a balance casing and at least one counterweight fastened to the balance shaft. Balance shafts have the objective of balancing the inertia forces and moments occurring in reciprocating machines. They are preferentially employed in light, high-speed internal combustion engines, in particular in pairs in engines with four in-line cylinders, for balancing the second order inertia forces. In the latter application, they are usually driven from the crankshaft at double rotational speed, i.e. at up to more than 10,000 rpm.
This means extreme requirements with respect to precision and bearing arrangements. At the same time, they should be as light as possible, cheap to manufacture and assemble and demand as little installation space as possible in the crank case. Two different designs are fundamentally possible — the balance shaft is either produced in one piece with its counterweights or it is “built up”, the counterweights being fastened onto the finished shaft.
The first possibility is for instance described in DE 37 05 346 A and the second in U.S. Pat. No. 4,425,821. The one-piece design is very expensive, demands maximum accuracy and leads, in the case of shafts with more than two bearings, to large bearing diameters for which, given the high rotational speeds, the lubrication technology cannot be successfully mastered. Precisely these reasons also prevent rolling contact bearings. On the other hand, built-up balance shafts have the main advantage of permitting smaller bearing diameters, it being however necessary to ensure sufficient shaft stiffness. In the case of more than two bearings, furthermore, single-piece casings with bearing bushes closed all round can also be used. It is, however, difficult to achieve maximum precision, reliable fastening and sufficient imbalance in the case of limited external dimensions. Thus in the case of the clamping connection of U.S. Pat. No. 4,425,821, for example, precision, minimum external diameter and connection strength is dubious.
The object of the invention is, therefore, to design a built-up balance shaft in such a way that, at minimum manufacturing cost, it meets the requirements of maximum precision, reliable fastening and minimum external dimensions.